1. Field of the Invention
The present invention generally relates to a so-called twin-clutch manual gearbox. More specifically, the present invention relates to a twin-clutch manual gearbox provided with an automated clutch between the engine and the manual gearbox for each gearbox speed grouping. The twin-clutch manual gearbox is useful for performing automated gearchanging by switching between engaging and disengaging these automated clutches (shifting control) and by alternating gearbox speed selection between both gearbox speed groupings.
2. Background Information
A conventionally known example of such a twin-clutch manual gearbox is the twin-clutch manual gearbox for the front engine, front wheel drive vehicle (referred to as an “FF vehicle”), as described in published Japanese Patent Application No. JP8-320054. The twin-clutch manual gearbox includes a first input shaft and a second input shaft to which engine rotation is selectively input via individual clutches. The hollow second input shaft is rotatably fitted onto the first input shaft, so that the first input shaft protrudes from the rearward end of the second input shaft, being that end farthest from the engine. Sets of gears (referred to as gearsets) associated with one gearbox speed grouping are provided between the rearward end of the protruding first input shaft and a layshaft of the gearbox located parallel to the first and second input shaft, such that appropriate transmission is enabled for respective gearsets. Gearsets associated with another gearbox speed grouping are provided between the second input shaft and the layshaft such that appropriate transmission is enabled for these respective gearsets, and such that rotation according to a selected gearbox speed after a gearchange is output in a radial direction from the forward end of the layshaft closest to the engine.
In such a twin-clutch manual gearbox, when a gearbox speed in one gearbox speed grouping is selected and the corresponding automated clutch is engaged, none of the gearbox speeds from another gearbox speed grouping can be selected. For a gearchange, a gearbox speed in another gearbox speed grouping is selected and, with the corresponding automated clutch disengaged, the automated clutch associated with said one gearbox speed grouping is disengaged while said automated clutch associated with said another gearbox speed grouping is engaged. This enables so-called clutch shifting control and enables automatic gearchanging by alternating gearbox speed selection between both gearbox speed groupings. In this way, automatic gearchanging is enabled on a manual gearbox.
In a twin-clutch manual gearbox for an FF vehicle, considering the convenience of gearbox assembly and because the rotation after gearchange is extracted in a radial direction from the forward end of the layshaft closest to the engine as described above, a favorable composition is to have the maximum diameter of the layshaft at its forward end, gradually decreasing the diameter approaching the rearward end.
However, because a twin-clutch manual gearbox for a front engine, rear wheel drive vehicle (referred to as an “FR vehicle”) must output the rotation corresponding to a selected gear after gearchange in an axial direction from the rearward end of the first input shaft or of the layshaft farthest from the engine, it is desirable for the convenience of gearbox assembly and layshaft strength for the layshaft to have a maximum diameter approximately midway (which is aligned near the rearward end of second input shaft), with the diameter gradually reducing from approximately midway toward both the forward end and the rearward end. For example, this construction means it is easier to assemble gearsets on the layshaft conveniently from both its rearward and forward ends.
The conventional layshaft in the twin-clutch manual gearbox for an FF vehicle, therefore, as described in Japanese Patent Application No. JP8-320054, is problematic for use as a layshaft in a twin-clutch manual gearbox for an FR vehicle.
To satisfy only the requirement for a layshaft with the maximum diameter approximately midway, and gradually decreasing in diameter toward the forward end from approximately midway, it would be sufficient to position the gearsets in high gearbox speed order from the forward side of the shaft closest to the engine, because the gearsets associated with the gearbox speed grouping provided between the second input shaft and the layshaft have a gear outer diameter that becomes smaller as the gearbox speed becomes higher. That is, a small diameter gear for a high gearbox speed is provided on the forward end of the layshaft closest to the engine, and the outer diameter of gears on the layshaft grow larger as gear distance from the engine increases. This therefore fulfils the requirement for a composition which puts the maximum layshaft diameter approximately midway, and gradually decreases the diameter toward the forward end from approximately midway.
Meanwhile, as also described in Japanese Patent Application No. JP8-320054, to support the first input shaft and the second input shaft rotatably fitted onto the layshaft, the gearbox housing is bearing the forward end of the second input shaft closest to the engine, and the gearbox housing is bearing the rearward end of the first input shaft protruding from the second input shaft. However, when the gearset associated with the lowest gearbox speed is positioned on the side farthest from the engine as described above, the gear which comprises said lowest gearbox speed gearset and is on the second input shaft has the smallest diameter. Such a gear restricts the size of the annular space between the first input shaft and the rearward end of the second input shaft, so much so that a needle bearing, or other bearing, cannot be retained in the annular space between the first input shaft and the rearward end of the second input shaft. Forming an annular groove in the first input shaft in which to place said needle bearing would cause decreased strength for the first input shaft, so this solution is not a desirable option. A possible alternative may be to move the needle bearing toward the engine to a position where no gears are present, but this would cause the needle bearing to approach the bearing between the forward end of the second input shaft and the forward end of the first input shaft, narrowing the bearing span and making the rigidity of the bearing between both input shafts somewhat deficient. Therefore this alternative solution is also difficult to implement.